This invention relates to threaded tuning mechanisms, and more particularly to method and apparatus for reducing backlash in and locking the position of a threaded tuning mechanism in the wall of a waveguide.
A variety of microwave devices such as frequency modulated oscillators (FMO's), amplifiers, multipliers, filters, etc. include a waveguide or other cavity supporting electro-magnetic waves. The cavity may be tuned with a metallic screw, probe or other sliding short circuit type of plunger that extends through a wall thereof. The operating frequency of a FMO, for example, is tuned by means of a threaded knob that is securely-rotatably mounted on the exterior of the enclosure. Rotation of the knob causes axial movement of a sliding short circuit inside the enclosure for changing the length of the resonant cavity and thereby adjusting the resonant frequency. It is desirable that threads in the tuning knob and on the plunger provide smooth and even movement of the plunger in opposite directions so as to provide continuous and even tuning of the device. It has been found that during and after the tuning process the axial position of the sliding short may change abruptly as the direction thereof is reversed in the cavity or from shock and vibration of the device such as occur when it is bumped or moved between work stations. It has also been found that precision fitting class 3 threads, for example, which are expensive and difficult to utilize on a production basis, do not totally compensate for backlash in the tuning mechanism. And as a practical matter, it is not possible to maintain perfectly fitting threads on a production basis. Set screws contacting the threads on the circumference of the plunger have also been used previously to lock the position of a tuning element, but this may damage the threads and produce an offset of the plunger which causes a frequency shift. Also, this does not compensate for backlash in the threads.
An object of this invention is the provision of an approved adjustment and locking mechanism for a pair of threaded elements.